Method of product control in suspension polymerizations



rmma Mar. 6, 19in METHOD OF PRODUCT CONTROL IN SUSPENSIONPOLYMERIZATIONS Robert C. Reinhardt, Midland, Mich., assignor to The DowChemical Company, Midland, Mich., a corporation of Delaware No Drawing.Application July 18, 1945,

Serial N0. 605,825

1 Claim.

for the monomeric substance and for the polymer products. It relates inparticular to an improvement in the suspension polymerization ofmonomers which normally pass from a liquid to a sticky semi-liquidagglomerative state and from thence to a non-sticky granular conditionin the process of their polymerization in a non-solvent liquid.

It is to be understood that the term suspension" is used herein todesignate dispersions of macroparticles which are maintained in asuspended condition in a non-solvent medium either by agitation alone orby a combination of agitation and a protective colloid, such as a gum ora cellulose ether which is soluble in the suspension medium. Thecommonest suspension medium is water. Others, such as the glycols, mayalso be used. The use of emulsifiers or surface active agents is notcontemplated in the process of the present invention.

It has'been recognized in my prior application, Serial No. 546,051,filed July 21, 1944, now Patent No. 2,445,970, that there are manypolymerizable liquids which may advantageously be polymerized in asuspension in a liquid non-solvent for both the monomer and the polymerand that such polymerization is often characterized by a transition ofthe dispersed macroparticles from a liquid to a sticky semi-liquidagglomerative state and from thence to a non-sticky granular condition.Among the monomeric materials which characteristically pass through thethree physical states listed above, in the course of theirpolymerization in a non-solvent liquid, are vinylidene chloride, vinylchloride, vinyl acetate, vinyl butyrate, styrene, acrylonitrile, estersof acrylic or methacrylic acids, and mixtures thereof with one another.

In the preparation of most polymeric substances, it is a primary objectto provide a product which is suitable for use in such typical plasticsworking operations as extrusion, injection molding, compression moldingand the like. For such uses, it is highly desirable that the plasticbeing supplied to the molding or extrusion apparatus be not only uniformin composition but also substantially uniform as to particle size. It isoften desirable that the feed be granular in character rather thanpowdery. In the typical emulsion polymerization of most monomericsubstances the polymer obtained is a fine powder which must often bereworked, in order to provide a granular feed for extrusion or moldingmoval from the polymer becomes practically imoperations. In the typicalsuspension polymerization heretofore known. wherein the material passesthrough the three successive stages outlined above, it has oftenoccurred that the polymer product varied in particle size from very finepowder to coarse granules or pellets and the instance has been known inwhich particles the size of eggs or larger have been found among thoseof smaller size in the product. It is believed that such non-uniformityof the particle size in suspension polymerizations is due to thenecessity of passing through the sticky agglomerative state during theprocess. If at this stage of the operation suflicient agitation isprovided to break up the individual particles, a grinding action occursand an excessive amount of powder, together with some larger pellets, isobtained as an end product. If the amount of agitation is reduced toavoid excessive powder formation the size of the pellets or granules mayincrease almost without limit. One means of solving this problem is setforth in my aforesaid copending application.

In spite of the above-mentioned disadvantages which have attached to theuse of the suspension method of polymerization certain advantages alsoattach to this method of operation. Chief among these is the fact thatthe product obtained does not contain internally bound or surface-borneemulsifying agent. Most emulsifying agents, not being thermoplastic incharacter, and having little, if any, softening or plasticizing actionon organic thermoplastics, perform no useful function in and ofteninterfere with such heat forming operations as extrusion or injectionmolding. During emulsion polymerization reactions the surface activeagents employed often become bound in the fine particles and theirrepossible.

It is accordingly an object of the present invention to provide a methodfor the suspension polymerization of liquid monomeric substances whichnormally pass from a liquid to a sticky semi-liquid agglomerative stateandfrom thence to a non-sticky granular condition during such apolymerization, without the necessity for passing through the saidagglomerative state. A related object is the provision of a method forthe suspension polymerization of monomers of the type above-identifiedwhereby there may be produced polymer particles of substantially uniformcomposition and particle size. A particular object is to provide amethod as aforesaid whereby there is produced a polymer, the uniformparticles of which are adapted to direct application to extrusion orplastics molding operations. It is a specific object of the invention toprovide such a method for the suspension polymerization of vinylidenechloride and of polymerizable mixtures containing the same.

The invention will be illustrated with particular reference to thepolymerization in aqueous suspension of vinylidene chloride monomer.When this monomer is suspended in water, suitably in the presence of asmall amount of benzoylperoxide, and is agitated at a polymerizationtemperature usually in the range of from about 25 to about 60 C., itpasses successively through the three physical states discussed above.It has been found that the suspended insoluble droplets of vinylidenechloride remain essentially liquid while the polymerization progressesfrom to about 5 per cent. Thereafter the suspended particles aresemi-liquid and sticky and tend to agglomerate into larger particleswhile the polymerization progresses from about 5 to about per cent. Theparticles then harden to a nonsticky granular condition which continuesthrough the balance of the polymerization process.

It has now been found that the foregoing and related objects may beattained by a method which comprises providing a suspension in anon-solvent liquid of a polymerizable substance in the non-stickygranular state and adding thereto a suspension of the correspondingmonomer at a temperature known to induce polymerization and at such arate that the suspension remains in the non-sticky granular condition.In the case of vinylidene chloride polymers the present process requiresthe provision of an aqueous or similar nonsolvent suspension ofnon-sticky granular particles which have been polymerized at least 15 toper cent and adding thereto a suspension comprising vinylidene chloridemonomer at a rate such that the degree of polymerization in thesuspension remains above 15 per cent. If desired the process may be madecontinuous by withdrawing a portion of the suspended polymer in amountcorresponding to the introduced suspension of monomer. While this may beaccomplished in any of numerous known types of polymerization equipment,it is preferred to carry out the reaction in a closed cyclic system inwhich the suspension may be circulating at a constant rate while thereis being introduced continuously a small amount of monomer, togetherwith water or similar liquid to suspend the monomer, and at the sametime there is being withdrawn from a point distant in the direction offlow from the point of introduction a corresponding amount of suspendedpolymer. In such a system, uniform particle size is assured by theconstant rate of circulation and by the continuous balance between feed,withdrawal and polymerization rate, i. e. the continuous avoidancetherein of conditions which might permit the formation of sticky andagglomerative particles. The initial charge, if desired, may bepolymerized in that system or it may be prepared by the simplesuspension in water of an appropriate amount of previously formedpolymer of the desired composition and particle size. The ratio ofsuspended particles to suspension medium may vary considerably, butgenerally lies between 1 and 0.2 for practical operation.

The success of the new method is dependent upon or arises from the factthat the freshly introduced monomer is absorbed in large part by theparticles of solid polymer already in the system about as fast as themonomer is introduced and there is thus no time at which sticky oragglomerative masses are present in the mixture. Since the suspendedparticles are solid at all time and show no tendency to adhere or sticktogether, the agitation may be adjusted to provide the amount ofgrinding action on the particles which will at the same time avoid theformation of particles which are too large and those which are too fine.It is thus possible to obtain, by filtration and drying of the withdrawnreaction product, a polymer which may be used directly as feed materialfor plastics working machines.

In a specific example, a suspension of 54 pounds of vinylidene chloridemonomer, 6 pounds of vinyl acetate monomer and 0.06 pound of benzoylperoxide in 200 pounds of water was heated with agitation in a glasslined kettle at 50 C. for 16 hours. At the end of this time theestimated conversion of monomer to polymer was about 25 per cent and themixture consisted of a suspension of hard granules. The suspension wasthen cooled, removed from the kettle, and loaded into a closed coil ofglass pipe 4 inches in. diameter and 44 feet in total length. Thesuspension of partially polymerized material was circulated through theglass coilat the rate of 4 to 5 lineal feet per second, which wassufficient to maintain the dispersion, and the temperature wasmaintained at about 30 C. by means of a water jacket around the glasspipe. After circulation of the charge had been established there wascontinuously introduced into the system an aqueous suspension consistingof about 92 per cent by weight of water and 8 per cent by weight of amixture of the monomers of vinylidene chloride and vinyl acetate in theproportions previously employed. The introduced aqueous suspensioncontained 1 per cent of henzoyl peroxide, based on the weight ofsuspended monomer. The rate of introduction of this suspension was about500 grams of water and 44 grams of monomer per hour. A similar amount ofsuspended granular polymer in its accompanying aqueous medium waswithdrawn continuously from a point in the coil 22 feet removed from thepoint of feed. From the known dimensions of the apparatus, it isestimated that only 0.5 per cent of the suspended product was beingwithdrawn each hour, and correspondingly only 0.5 per cent of thesuspension was newly introduced within the hour. After operating for 50hours, circulation was stopped and the coil was emptied. The total yieldof polymer, all of which was a uniform granular, non-sticky product, was34.4 pounds, including that collected during operation of the coilpolymerizer. After allowing for the polymer originally produced in thekettle, the polymerization rate in the coil was found to be 0.6 per centper hour. The dried polymer was found to be satisfactory for use as feedmaterial, without purification, in a standard extrusion machine. Theaverage extent of polymerization had increased in the coil from aninitial 25 per cent value to a final value near 70 per cent. This meansthat both feed and withdrawal could be at a somewhat greater rate thanthat employed (up to 5 times as great), other factors remainingconstant, without running into any danger of agglomeration, since thesticky state would not be encountered.

In commercial operation, the process would normally be continuedindefinitely, and two or more similar coils might be operated in series,

a small portion of partial polymer being continuously withdrawn from thfirst and used as feed for the second coil, wherein polymerization wouldcontinue more nearly toward completion. The use of such a multiple coilpolymerization system is described and claimed by R. H. Boundy inapplication Serial No. 545,988, filed July 21, 1944 (now abandoned) andin a continuation-in-part thereof, Serial No. 710,469, filed November18, 1946, now abandoned.

The invention has been illustrated with reference to continuousoperation in a coil or series of coils, but is by no means limited tosuch type of operation. Thus, in a batch type of procedure, a suspensionof non-sticky partially or fully polymerized particles may be preparedin a non-solvent liquid, suitably in a kettle fitted with a stirrer, andthere may be added thereto an amount of monomer such that the averagedegree of polymerization in the kettle always exceeds the maximum amountfrom which sticky, agglomerative particles are obtained. The monomer maybe introduced continuously or in small batches, relative to theinventory of polymer in the kettle, and may be introduced alone or mixedwith the non-solvent suspension medium, and should contain catalyticquantities of benzoyl peroxide or other suitable accelerator. Thesuspension of polymer may likewise be allowed to accumulate in thekettle, or it may be removed continuously or in small batches. It isoften desirable, in batch operation, to build up gradually an inventoryabout equal to the working capacity of the reactor, and then todiscontinue monomer addition and to continue agitation at apolymerization temperature until substantial completion of the reaction.A uniformly effective degree of agitation must be maintained throughoutthe polymerization, to provide a product of uniform particle size.

If desired, there may be added to the suspension, during polymerization,a plasticizer for the polymer in amount to provide the desired finalplasticized composition. The usual plasticizers are insoluble in theaqueous or other non-solvent media used in suspension polymerizations,and suspended droplets of the plasticizer become absorbed by or adsorbedon the suspended polymer particles. In like manner, if desired, theremay be added suspensions of other such modifying agents as heat andlight stabilizers, oil-soluble dyes, and the like.

When operating in accordance with th present invention, the usualconditions of temperature, and the customary catalysts are employed forthe particular monomer or mixture of monomers be-- ing subjected topolymerization. The invention does not reside in the selection or theseconditions, but rather in th controlled addition of monomer to thesuspension at such a rate as to avoid the sticky, agglomerativecondition in the suspended particles, i. e. at such a .rate as tomaintain in the suspension an average condition typical of the thirdstage in such polymerizations, wherein the suspended particles arenon-sticky granules. The actual ratio of monomer to polymer at whichthis transition from sticky to dry particles occurs is different foreach monomeric material, but in the case of vinylidene chloride and itscopolymers this ratio does not exceed 4, i. e. there should be not morethan 4 parts of monomer for each part of polymer, to avoid the stickystate.

I claim:

In the suspension polymerization Of vinylidene chloride and mixturesthereof with a monomer selected from the group consisting of vinylchloride, vinyl acetate, vinyl butyrate, styrene, acrylonitrile, estersof acrylic acid and esters of methacrylic acid, wherein vinylidenechloride predominates, and which normally pass during polymerizationfrom a liquid to a sticky agglomerative state and thence to a non-stickygranular condition, the steps which include: maintaining apolymerization temperature in a constantly agitated aqueous suspensionof from 0.2 to 1 part by weight of non-sticky granular particlesconsisting of at least 20 per cent of the desired polymer, any balancebeing essentially the corresponding monomer, for each part by weight ofwater, and in the absence of an emulsifying agent; adding thereto asuspension of the correspondin monomeric material and a polymerizationcatalyst, in water, in a ratio and at a rate to maintain in the systemnot over 4 parts of monomer for each part by weight of polymer, whilewithdrawing therefrom suspended polymer ata rate equivalent to the rateof monomer feed, thereby maintaining a steady state in the system and anon-sticky granular condition in the disperse particles throughout theirresidence in the polymerization zone; and recovering the non-stickypolymer particles from the withdrawn portion of the reaction mixture.

ROBERT C. REINHARDT.

REFERENCES CITED The following references are of record in the flle ofthis patent:

UNITED STATES PATENTS Number Name Date 2,171,765 Rohm et a1. Sept. 5,1939 2,380,009 Arnold et a1. July 10, 1945 2,445,970 Reinhardt July 27,1948

